Question: Gabriela is 18 years older than Christopher. Eighteen years ago, Gabriela was 3 times as old as Christopher. How old is Christopher now?
Answer: We can use the given information to write down two equations that describe the ages of Gabriela and Christopher. Let Gabriela's current age be $g$ and Christopher's current age be $c$ The information in the first sentence can be expressed in the following equation: $g = c + 18$ Eighteen years ago, Gabriela was $g - 18$ years old, and Christopher was $c - 18$ years old. The information in the second sentence can be expressed in the following equation: $g - 18 = 3(c - 18)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $c$ , it might be easiest to use our first equation for $g$ and substitute it into our second equation. Our first equation is: $g = c + 18$ . Substituting this into our second equation, we get the equation: $(c + 18)$ $-$ $18 = 3(c - 18)$ which combines the information about $c$ from both of our original equations. Simplifying both sides of this equation, we get: $c + 0 = 3 c - 54$ Solving for $c$ , we get: $2 c = 54$ $c = 27$.